This invention relates to a screw feeder for conveying material in a definite direction irrespective of the rotation direction of the screw feeder. The screw feeder according to the invention is intended primarily for feeding cellulose-containing material to a grinder (disc refiner), but it can also be used for feeding other material to apparatuses of other types. It is imaginable, for example, to use the screw feeder for feeding coal to a boiler. The invention generally is applicable when a reversible feed screw is desired and at the same time the feed direction is to be maintained.
Reversible screw feeders with adjustable thread elements are per se known. The thread elements are designed as wings extending 180.degree. about the screw axle. Each wing is rotatable on an axle extending radially in relation to the screw axle. A plurality of such wings are arranged one after the other on the screw axle, and by rotating the wings about their radial axles it is possible to adjust the wings so that they together form either a right-hand thread or a left-hand thread. Such an arrangement is disclosed, for example, in U.S. Pat. No. 1,320,214.
A screw feeder of this kind, however, has a restricted application range, because the construction is weak and the adjusting is complicated. It cannot be used, either, at a high number of revolutions of the screw axle.
In order to further elucidate the invention, the background of the invention is discussed in the following with reference to the grinding of lignocellulose-containing material in a grinder of disc-refiner type for the production of fibre pulp.
This technique implies a mechanical treatment of the material between opposed refiner discs, which both rotate in opposed directions, or one of which is stationary and the other one rotates. These refiner discs are provided with refining surfaces comprising bars and grooves for processing the material, which is supplied through one refiner disc and moves outward between the refiner discs by action of the centrifugal force.
The refining surfaces usually are formed on detachable refiner elements, which are mounted on the refiner discs. During the processing of the material, the bars on the refiner elements, especially the leading edges of the bars seen in the rotation direction of the refiner discs, are gradually worn.
For compensating this wear, it is desirable to turn the rotation direction of the refiner discs in intervals of operation hours. Hereby the previous leading edge of the bars is replaced by the less worn trailing edge. This method can be carried out in cases where the material is advanced to the refiner discs by a separately driven feed device of screw type which, for example, can be mounted concentrically about an axle carrying a refiner disc.
In certain cases, however, it is very difficult or impossible to use a concentrically mounted feed device, because such a device is relatively complicated and requires much space. This applies especially to refiners with opposed or through rotation axles. The feed-in there is effected through a screw thread, which is fixed stationary on the axle of the refiner disc. This screw thread, thus, rotates with the refiner disc and thereby renders it impossible to revers the rotation direction of the refiner disc. As pointed out above, this results in that only one side of the bars on the refining surfaces are worn. Hereby the service life of the refiner elements is shortened, which implies increasing costs for the pulp manufacture.
Another way of solving this problem is to mount stationary radial members with triangular cross-section on the rotary axle of the refiner disc. Upon rotation of the axle in either direction, the flanks of these members subject the material supplied to momentary impacts and thereby cause the material to move to the feed-in opening of the refiner disc. Such a jerky feed yields a very low feed-in effect compared to a continuous advancing flank of screw thread type.
The present invention has the object to produce a screw feeder with a reversible feed screw having a continuous feed flank of screw shape. The material is to be advanced in a definite direction irrespective of the rotation direction of the feed screw.